A wide variety of roller cone or rotary cone drill bits, which are a type of rock bit have been and are currently being used to form wellbores or boreholes in subterranean formations for oil, gas, geothermal stream, and the like. Roller cone drill bits generally include at least one support arm and often three support arms. A respective cone assembly may be rotatably mounted on a spindle or journal extending inwardly from an interior surface of each support arm. Although many specific examples in the current specification refer to roller cone drill bits, it is well-known that grease compositions usable in roller cone drill bits are often usable in other rock bits as well.
A cone assembly typically includes a cavity with a configuration and interior dimensions sized to receive exterior portions of an associated spindle therein. A wide variety of bearings, bearing assemblies or other supporting structures may be disposed between interior portions of the cavity of each cone assembly and exterior portions of an associated journal or spindle. Grease is filled into the cavity of this roller cone assembly to provide the lubrication required to separate the moving parts. Surface coatings (such as silver) are engineered onto the bearing surfaces to protect the surfaces. Fluid barriers include seal material or diaphragms. Seal material may include hydrogenated nitrile rubber (HNBR) and may be used to seal and prevent drilling mud from entering into the roller cone assembly.
Such a roller cone drill bit, during the downhole drilling, is subjected to severe operating conditions such as constant and repetitive shock loads exerted to the interior portions of bearing surfaces with pressure in excessive of 15 MPa and temperature above 150° C.
In order to sustain the useful life of such roller cone assembly including journals, bearings, seals and fluid barriers, and surface coating materials and subsequently extend the duration of the drilling operation, one uses a rock bit grease.
Grease is a semi-solid comprising base oil, additive, and thickener dispersed within to give the gel-like texture. Because greases do not flow readily, they are used where extended lubrication is required and where oil would not be retained. One such example is the soap grease that is formed by reacting (saponifying) a metallic hydroxide, or alkali, with a fat, fatty acid, or ester in the presence of base oil and necessary additives. The selection on the type of soap grease used depends on the grease properties desired. For instance, calcium (lime) soap greases are highly resistant to water, but unstable at high temperatures. Sodium soap greases are stable at high temperatures, but wash out in moist conditions. Lithium soap greases resist both heat and moisture. A complex soap grease is an improved version of soap grease that is formed by combining multiple sources of acids such as high-molecular-weight fat or fatty acid, short chain acid, dibasic acids, or inorganic acid, and the alike, in the grease manufacturing that results in the increase the dropping point of grease. Lithium, calcium, and aluminum greases are common alkalis in complex soap grease. Non-soap thickeners, such as clays, silica gels, carbon black, polyurea and various synthetic organic materials are also used in grease products. A mixed-base grease is a grease that utilizes different types of thickeners such as soap and non-soap thickener.
A suitable rock bit grease for roller cone bits typically fulfills a number of requirements. First, it may be capable of meeting lubrication requirements such as load carrying and wear reduction under the extreme high pressure and load conditions during subterranean drilling. Second, the grease may be compatible with fluid barriers and surface coatings to eliminate or reduce leakage and to maintain the integrity of the metal surface of the roller cone bits. Third, in downhole drilling, the pressure differential between the drilling well bore and the bit interior is usually carefully controlled regardless of the mud overhead placed on the drill bit. Maintaining this pressure differential within a specified limit ensures reliable performance of the primary seal in a drill bit. Any pressure variations received from the bore well ideally propagate through all internal bearing/seal components with minimal loss and time delay. The media responsible for transferring/propagating pressure signals as such is the grease. Accordingly, greases may be designed to exhibit rheological properties such as grease fluidity and consistency over the entire life and fast-changing operating environment of the demanding drilling operation.
Failures of the grease to deliver all of the functions mentioned above, namely, abilities to lubricate and reduce wear, seal and surface material compatibility, anti-corrosiveness, and adequate rheological responses, will result in the premature failures of the roller cone assembly and ultimately failures of the entire drilling operations which can not only cost dearly and significantly more than the cost of replacing defective parts but also consume precious time and energies to retrieve the failed bits and to resume drilling operations.
A wide variety of grease compositions have been employed and proposed in roller cone drill bit or rock bit applications. For instance, Smith U.S. Pat. No. 4,358,384 describes examples of greases for roller cone drill bits with a petroleum derived base stock and metal soap or metal complex thickener.
Calcium (Hughes Tool, U.S. Pat. No. 3,935,114) and Calcium complex greases (Dresser, U.S. Pat. No. 4,409,112) are known in arts to lubricate rock bit with good load carrying and wear reducing properties.
Smith U.S. Pat. No. 5,668,092 discloses high viscosity base oils in thickener such as silica, which are claimed to have excellent film strength in lubrication.
Baker Hughes U.S. Pat. No. 5,891,830 discloses Calcium complex synlubes, which are claimed to have improved lubrication and thermal stabilities.
Tomlin Scientific U.S. Pat. No. 7,312,185 discloses the use of high-viscosity poly-alpha-olefin (PAO) and other synthetics such as alkylated naphthalene to enhance the thermal stability of the grease.
Despite these advances, the lubricating greases for roller cone drill bits have focused primarily on the improved lubrication through the use of novel extreme pressure (EP) additives, and enhanced thermal abilities through the use of synthetic base oils. Further advances are needed that address rheological aspects of greases that can be beneficial to the operation of the roller cone drill bits. In particular advances over traditional complex soap grease and/or non-soap thickener systems such as silica are needed. In particular, greases with even higher stability, enhanced lubrication, improved compatibility with fluid barriers and surface materials, and/or improved anti-corrosion properties as well as improvements in rheological properties such as the ability to transfer pressure signals over the entire drilling operation are needed. Improvements in all or several of the above properties will enhance the durability and the life of the roller cone drill bits.
One aspect of current disclosure relates to greases with such improvements. Particular examples relate to greases containing sulfonates. Sulfonate greases are known to provide outstanding mechanical, anti-corrosion, and good inherent load carrying properties that are related to the calcite or micelle like structure of the grease (see e.g. U.S. Pat. No. 4,560,489 of Witco Chem. Corp.; U.S. Pat. No. 5,126,062 of NHC Corp.; Matthew Sivik and Bill Ward, The Lubrizol Corporation, “Understanding Calcium Sulfonate Thickeners,” Machinery Lubrication Magazine, July 2006). A weld load of 400 kgf or even higher determined by four-ball weld bench test is common with calcium sulfonate grease. Calcium sulfonate grease was also noted as having a higher dropping point than those greases based on traditional complex soap grease thickeners.
U.S. Pat. No. 7,294,608 of Jet-Lube discloses calcium sulfonate greases as the threading compounds for oil field applications that operate in contact with drilling mud. These greases are claimed to have significantly improved resistance toward corrosion, erosion, ablation benefits and protection of threaded connection surfaces.
These calcium sulfonate greases are untested in roller cone drill bits or in conditions roller cone drill bits are likely to encounter, or these greases leave additional room for improvement in one or more grease properties discussed above.